In an existing engine, a temperature of cooling water inside a cylinder head and a cylinder block are controlled by a cooling water control mechanism positioned at an engine inlet or an engine outlet. By this, the cylinder head and the cylinder block maintain the cooling water temperature. However, in order to improve fuel efficiency and performance, a variable dividing cooling technology separately controlling the cooling water in the cylinder head and the cooling water in the cylinder block has been developed recently.
During an engine operation, a piston friction loss accounts for the highest ratio among engine friction resistances. Upon increasing a temperature of a wall surface of the cylinder block to prevent piston friction, the piston friction loss decreases, and thus, fuel efficiency is improved. On the other hand, upon increasing the overall temperature of the engine to increase the temperature of the cylinder, combustion stability such as knocking becomes problem. For this reason, as a method for lowering a temperature of a head portion of a combustion chamber while maintaining the temperature of the cylinder block high, a variable dividing cooling technology for controlling the cooling water of the cylinder head and the cooling water of the cylinder block, respectively, are implemented to secure the combustion stability while achieving the improvement in fuel efficiency. That is, the block blocks a flow rate of cooling water at a medium speed or less and a heavy load or less in an engine operating area to keep the temperature high and the head side maintains the temperature as before or slightly reduces the temperature but increases a flow rate of cooling water of the block side at a high-speed, high-load operation to lower the temperature of the cylinder.
To apply the variable dividing cooling technology, a structure divide the cooling water of the cylinder head and the cooling water of the cylinder block is required, which is generally implemented to remove water holes of a head gasket. However, for this purpose, there is a need to increase the overall temperature of the cylinder block. In this case, a temperature of a siamese portion of the block and a temperature of a piston top ring portion excessively rise upon combustion, and therefore, knocking characteristic is aggravated. Further, there is a limit of increasing the temperature of the block, in a low and medium-speed, high-load area, such that the fuel efficiency may be decreased and the durability may be aggravated.